It is called an unreactive element because it has a stable electronic configuration and does not share its electrons with other atoms.
Argon, discovered in 1894, makes up 0.93% of the earth's atmosphere, which makes it the third most abundant gas. It is a byproduct of the production of oxygen and nitrogen.
Argon is used to fill incandescent and fluorescent light bulbs to prevent oxygen from corroding the hot filament. It is also used in arc welding, growing semiconductor crystals, and "processes that require shielding from other atmospheric gases."
Visit the links below for more information, especially the site scienceonline.org, from which most of my info cam.